In the case where a measurement of light reflected by or transmitted through a sample is performed with a spectrometer, if the light is diffuse light, an integrating sphere is used to maximally introduce the light from the sample into a detector. An integrating sphere has a light entrance window and light detection window. The light from the sample is introduced through the light entrance window into the integrating sphere. This sphere internally has a reflection surface with a high level of reflectance. The light which has entered the integrating sphere is reflected by the inner surface of the integrating sphere, to eventually pass through the light detection window and fall onto the detector (a portion of the light directly reaches the detector). In the case where the irradiation of a sample with light is performed via the integrating sphere, a light introduction window is additionally formed in the integrating sphere, allowing light from an external source to be introduced through this light introduction window into the sphere and cast onto the sample.
In the case of performing a spectrometric measurement over a wide range of wavelengths, two or more types of detectors having different wavelength-sensitivity characteristics are normally used. Conventionally, two types of detectors have been used for such purposes, i.e. a PMT detector, which has a high level of sensitivity to ultraviolet-visible light, and a PbS detector, which has a high level of sensitivity to near infrared radiation.
FIGS. 1 and 2 show an integrating sphere 1 with a light introduction window, which allows for the use of two types of detectors and has the following elements arranged: a light introduction window 2, light entrance window 3, two light detection windows 10 and 11, as well as two detectors 10a and 11a. As shown in FIG. 1, a light entrance window 3 is formed in the integrating sphere 1. With the straight line connecting the center of the integrating sphere 1 and the light entrance window 3 defined as the X axis, and one arbitrary straight line which passes through the center of the integrating sphere 1 perpendicularly to the X axis defined as the Z axis, the light detection windows 10 and 11 are placed at the points of intersection of the Z axis and the integrating sphere 1 (these points are hereinafter called the “polar points”), with the detectors 10a and 11a placed on the outside of the light detection windows. With the axis passing through the center of the integrating sphere 1 perpendicularly to both of the X and Z axes defined as the Y axis, the light introduction window 2 is placed at a position on the X-Y plane opposite from the light entrance window 3 with respect to the center of the integrating sphere 1, being slightly displaced from the point of intersection of the X axis and the integrating sphere 1. This displacement is made in order that the directly reflected (mirror-reflected) ray of the light which is reflected by the surface of the sample 4 and enters the light introduction window 2 to the outside.
The light which has been generated from the light source and has entered the integrating sphere 1 through the light introduction window 2 is cast onto the sample surface through the light entrance window 3. After undergoing absorption at specific wavelengths to the sample, the light is reflected through the light entrance window 3 into the integrating sphere 1. After being reflected on the inner surface of the integrating sphere 1, the light falls onto the detectors through the light detection windows (a portion of the light directly reaches those detectors). The intensity of the reflected light changes with the reflection angle, with its intensity distribution expressed as a cosine distribution which is symmetrical with respect to the X-Y plane. Since the two types of detectors 10a and 11a located at the two polar points have symmetrical detection fields with respect to the X-Y plane, the detection surfaces of the detectors 10a and 11a receive light from the detection fields which have equal intensity distributions of the diffuse reflected light. Therefore, when the measurement of the diffuse reflected light is performed with the wavelength of the incident light gradually changed, no discrepancy in the detection result (“measurement discrepancy”) occurs when the detectors 10a and 11a are switched taking into account their wavelength-sensitivity characteristics.